Page 319 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

11.6 Flash distillation 321

Deliverables from the process design phase forming the basis for mechanical design of a typical
flash distillation system are

• General arrangement of the system, its instrumentation and control e P&ID.
• Operating temperature (T) and pressure (P) of flash drum.
• Flow rate and composition of vapour and liquid streams leaving the drum.
• Feed preheat temperature and preheating load, if preheating is required. Heating coil/jacket
details in the drum and the heat load, if required.
• Drum dimensions, internal fitting like demister pad, vortex breaker and nozzle connections and
their locations. Requirement of vacuum, if operation is envisaged under vacuum.

11.6.1 Design equations
Design equations related to the process streams
Assuming ideal system, the distribution coefficient of component i,
sat
i
p ðTÞ

i
P
K i ¼ y =x i ¼
p sat is the vapour pressure of the i-th component at temperature T and pressure P.
i
For nonideal systems: K i ¼ g p sat P , the liquid-phase activity coefficient g can be calculated for
i
i i
known temperature and composition. These calculations are straightforward and are dealt in details in
text books on thermodynamics.
In case of low pressure systems, where the pure component vapour pressure is below 2 atm, the
vapour pressures for component i can be estimated from Antoine’s equation:
sat b i
log 10 p i ¼ a i (11.32)
ðT þ c i Þ
Antoine constants a, b and c for component i are in corresponding units based on the units of
sat
T and p .
i
When (V/F) is the fraction of moles of feed leaving the drum as vapour and z Fi denotes the mole
fraction of the i-th component in feed for an n component system.
n
X
¼ 0 (11.33)
z Fi ðK i 1Þ
1 1 þðV=FÞ ðK i 1Þ
The above equation can be solved for ðV=FÞ using an iterative method.
Drum pressure P being between the bubble and dew point pressure P bub and P dew is the essential
condition for existence of both phases in the drum. Only one positive (V/F) value between 0 and 1 will
be obtained as a solution of the aforementioned equation. Once the (V/F) value is found from
Eqn. 11.33, the liquid and the vapour compositions can be found from the following equation
z Fi
; (11.34)
x i ¼
1 þðV=FÞð1 K i Þ
where x i and y i ð¼ x i K i Þ are mole fractions of i-th component in the liquid and vapour phase.